Synthesis of Five-Membered Osmacycles with Osmium–Vinyl
Bonds from Hydrido Alkenylcarbyne Complexes
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Abstract
Treatment of the osmium hydrido butenylcarbyne
complex [OsH{CC(PPh<sub>3</sub>)CH(Et)}(PPh<sub>3</sub>)<sub>2</sub>Cl<sub>2</sub>]BF<sub>4</sub> (<b>1</b>) with
excess 2-chloro-4-cyanopyridine in the presence of H<sub>2</sub>O<sub>2</sub> generates the fused osmacyclopentadiene <b>2</b>. A
detailed mechanism of the conversion has been investigated with the
aid of in situ NMR experiments and the isolation of intermediates <b>3</b> and <b>4</b>. In contrast, reaction of <b>1</b> with the propiolic acid ester HCCCOOMe produces the osmafuran <b>5</b>. Analogous reactions of the osmium hydrido phenylethenylcarbyne
complex [OsH{CC(PPh<sub>3</sub>)CH(Ph)}(PPh<sub>3</sub>)<sub>2</sub>Cl<sub>2</sub>]BF<sub>4</sub> (<b>6</b>) with
nitriles/CO or HCCCOOMe were also studied, which result in
the formation of five-membered osmacycles [Os{CHC(PPh<sub>3</sub>)CH(Ph)(OH)}(CH<sub>3</sub>CN)(PPh<sub>3</sub>)<sub>2</sub>CO](BF<sub>4</sub>)<sub>2</sub> (<b>7</b>) and [Os{CHC(PPh<sub>3</sub>)CH(Ph)(OH)}(PhCN)(PPh<sub>3</sub>)<sub>2</sub>CO](BF<sub>4</sub>)<sub>2</sub> (<b>8</b>). In the presence of NEt<sub>3</sub>, <b>6</b> can convert
to the osmium hydrido phenylethenylcarbyne complex OsH{CCCH(Ph)}(PPh<sub>3</sub>)<sub>2</sub>Cl<sub>2</sub> (<b>9</b>) in wet acetonitrile,
presumably involving P–C bond cleavage. Similarly, <b>9</b> can react with HCCCOOMe with the aid of HBF<sub>4</sub> to
give osmafuran <b>10</b>
